Here's a problem that's been driving me nuts, and I finally reduced it to a small self-contained script which can be found at http://gist.github.com/642538. The issue is that the pick_event does not always provide the correct index into the plotted data.
In case this script only bugs out on my machine, let me describe the problem. The callback should highlight whatever point is clicked. This happens correctly with no problems when clicking on 9 out of the 10 points in the figure. But clicking the indicated point on the left-hand side (x, y) also causes the indicated point on the top right (x, y) to be highlighted as well.
Frustratingly, it seems that the numbers themselves are important for reproducing this bug! Specifically, x+=0.01 does not "fix" the bug . . . but x += 0.1 makes the callback work correctly (i.e. only the clicked point is highlighted). I'm guessing this is some sort of tolerance issue somewhere, but I can't find it.
Ubuntu 10.04 64bit